In the medical treatment of patients requiring breathing assistance it is common to insert an endotracheal tube into the trachea of the patient by way of the mouth or nose or by way of a surgically created opening into the patient's trachea. The rear or proximal end of the endotracheal tube is connected to a breathing apparatus, such as a respirator, which will periodically force air into the lungs through the tube. The inner or distal end of the tube is typically provided with an annular, inflatable cuff which is inflated after the tube is inserted into the patient's trachea to provide a seal against the interior wall of the trachea. The cuff is inflated by means of a conventional syringe which forces air into the cuff through a relatively small diameter secondary tube, known as a lumen or tubule, provided in the wall of the endotracheal tube.
By way of example, endotracheal tubes provided with inflatable, trachea-sealing cuffs on the inner or distal end thereof are shown in U.S. Pat. Nos. 3,402,718; 3,504,676; 3,565,079; 3,642,005; 3,693,624; 3,794,036; 3,794,043; 3,848,605; and 3,901,246.
One of the problems associated with the use of an endotracheal tube having an inflatable, trachea-sealing cuff on its distal end is that there is a tendency to over-inflate the cuff so that it may press too tightly against the tracheal wall. Over-inflation causes discomfort to the patient and also can cause such serious complications as dilation of the trachea, blockage of blood circulation and necrosis of that portion of the trachea around the cuff.
Various ways have been proposed to prevent over-pressurizing of such cuffs. For example, U.S. Pat. No. 3,642,005 discloses an endotracheal tube with an inflatable cuff and a pressure-regulating balloon connected to the cuff. The balloon is of such a character that it will expand to a size at which the air pressure in it is the same as the maximum air pressure desired in the cuff, after which the balloon will expand further without increasing that maximum pressure as additional air is forced into the cuff. However, the pressure-regulating balloon of the U.S. Pat. No. 3,642,005 gives rise to another problem. The breathing apparatus (e.g., a respirator) connected to the endotracheal tube sometimes intermittently delivers air through the endotracheal tube and to the patient at such high pressures that it compresses the cuff and forces air back into the balloon, thereby breaking the seal between the cuff and the trachea wall so that air delivered by the breathing apparatus can escape between the cuff and the interior wall of the trachea. In an effort to overcome this problem, U.S. Pat. No. 3,794,043 proposes the use of a check valve between the pressure-regulating balloon and the inflatable cuff. The check valve of the U.S. Pat. No. 3,794,043 closes when there is an increase in air pressure at the distal end of the cuff (such as may occur every time the breathing apparatus forces a charge of air or other gas into the lungs) to prevent air from the cuff from being forced back into the balloon. However, the device of the U.S. Pat. No. 3,794,043 gives rise to another problem. Air (or other gas) from the breathing apparatus which is connected to the endotracheal tube can migrate through the material of the inflatable cuff and thereby over-pressurize the cuff. It is desirable to relieve this over-pressurization of the inflatable cuff. However, since the check valve of the U.S. Pat. No. 3,794,043 closes when the pressure in the inflatable cuff exceeds the pressure in the regulating balloon, the cuff remains overpressurized.